In art, this invention is believed to be in class 340, subclass 649.
Industrial machinery often has internal wiring and components which operate with high voltages that are dangerous to people. Protection is accomplished by covering the components where high voltage exists with electrical insulation and then enclosing the high voltage components with protective metal covers or cases. All of the exterior metal parts of the machinery are then connected directly to safety ground. If any of the insulation around the high voltage components deteriorates or is broken, and a current path from the high voltage to the surrounding metal exists, the connection to safety ground is capable of carrying enough current to open the fuses or circuit breaker supplying power to the system. This prevents high voltage from appearing on any exterior metal part of the machine, where a person could touch it.
To assure protection against electrical shock, it is necessary to constantly check the connection between safety ground and the machine containing high voltage. The usual way is with a low-ohm resistance measurement.
However, certain types of industrial machinery operate in a way which causes unknown amounts of current to intermittently flow in the safety ground circuit. This current in the safety ground circuit happens during normal operation and it is not practical to eliminate it. Although these unknown currents are almost always low enough to not create any safety issues, they do interfere with the measurement of the resistance between the machine and safety ground.
The invention is comprised of a set of methods together with apparatus to detect the resistance of a safety ground circuit and to assure that no voltage harmful to humans exists on the safety ground circuit or the devices which are connected to the safety ground circuit. If the resistance of the safety ground circuit rises above a predetermined value or a harmful voltage is found anywhere on the safety ground circuit or an attached device, outputs are provided to indicate an unsafe condition and to cause power to be removed from the devices which contain high voltage and are connected to the safety ground circuit.
The invention is distinguished from the prior art by the ability to detect the resistance of the safety ground circuit even if intermittent and unknown currents are flowing in the safety ground circuit. The invention is further distinguished from the prior art by the measurement of voltage potential to detect hazardous voltage all of the time, even when the intermittent and unknown currents make resistance detection impossible. The invention uses only simple switched DC for resistance detection and does not require any sort of external sensors to measure things like current or temperature.
The resistance of safety ground is normally in the range of 0.1 to 10 ohms. This is because the resistance path from any point of the grounded system back to safety ground must be capable of handling enough current to quickly cause the circuit protection device (fuse or circuit breaker) feeding power to whatever is enclosed by the grounded cabinet, housing, cover or case to open if a short-circuit between incoming power and the enclosure occurs. Otherwise, the incoming power voltage would appear for significant time in places that could be touched by people. In many industrial devices, the incoming power feed is at a voltage of approximately 500 volts and is fused at hundreds of amperes. Safety ground resistances no more than a few ohms are required.
Within a building, safety ground is ultimately earth ground. Earth ground is achieved by driving one or more metal grounding rods 1 meter or more into the ground to establish electrical contact with the underlying soil. In a building with a steel frame, where the steel supporting members contact the ground as part of the foundation structure, the grounding rods are connected to the steel frame of the building to improve the electrical connection to the underlying soil.
Inside the building, where machinery is installed, there is usually a separate conductor, made of copper or aluminum which is used only for safety ground connection. This safety ground conductor is usually routed to one or more grounding rods and is also connected to the steel frame (where present) of the building at many points throughout the building. The safety ground conductor carries no powerline current and is only tied to the neutral point of the incoming power at a single point. The purpose of the safety ground is to provide a place to connect the metal protective barriers (such as cabinets and housings) which surround high voltage components to earth ground.
In an industrial environment, there are many things which could cause the safety ground to become disconnected. Ground cables attached to moving machinery may break due to metal fatigue in the constantly-flexing cable. Moving machinery or fork lift trucks might snag on cables and break them. Ground cables may be disconnected during repair or maintenance. In places where corrosive vapors and/or moisture are present, corrosion at joining points may occur.
The goal of this invention is a simple, reliable means to detect a broken safety ground connection where an apparatus connected to safety ground may cause an intermittent and unknown current, sufficient to disrupt a resistance measurement, to flow in the safety ground circuit.
All of the blocks in the flowcharts of FIGS. 3 and 4 are numbered. Where a block is identical in both figures, it carries the same number.